When irradiated nitrogen-rich natural diamond was heated isothermally, the vacancies were destroyed at two rates. In both processes vacancies migrate to and were trapped at the nitrogen. The faster process coincides with a sharpening of the optical transitions of the vacancy. Both processes have the same activation energy of Em = 2.3eV. By annealing nitrogen-free diamond, Em was shown to be the migration energy of the neutral charge state of the vacancy.

Migration Energy of the Neutral Vacancy in Diamond. Lawson, S., Davies, G., Collins, A.T., Mainwood, A.: Journal of Physics - Condensed Matter, 1992, 4[9], L125-31